Ducts of the type briefly defined in the foregoing are shown, for example, in German Gebrauchsmuster (utility model) No. 7,308,228. In that proposal an outer annular shoulder or abutment, formed integrally with the duct tubing, serves to form an opening which in turn receives a packing or seal and a retaining ring for the retention in fixed position of the seal or packing. Provision is thus made for the relative positioning and the positive sealing of inserts or retainers with which the cables, pipes or the like are positioned in the duct tubing.
In order to ensure proper sealing, the outer surface of the duct tubing is coated with a porous and elastic material which, on the one hand, will adhere to the tubing while, on the other hand, it will allow concrete, cement or the like to permeate to the extent required to effect an integral bond between the duct tubing and the final wall structure, which bond will not be adversely affected by drying or settling of the surrounding material.
The seal provided in this prior art proposal does not, however, significantly contribute to such a bond since it cannot be compressed axially.
In general, the duct according to this prior art proposal has been accepted relatively well in its respective market. However, it directs attention to the requirement that a seal means must be provided between the duct tubing and the wall structure that will withstand, under conditions of varying humidity and pressure of gases, as are normally prevailing in a environment in which it will be employed, these adverse conditions. That is to say, the seal must be effective against pressures that exceed 1/10 atmospheres. The structure of the prior art devices does not meet this requirement and can therefore be utilized in depths below 1 meter below the groundwater level only conditionally.